Posted
by
timothyon Saturday March 24, 2012 @06:50AM
from the tell-me-when-we-find-huge-and-edible-creatures dept.

An anonymous reader writes with this selection from a press release issued by the Harvard-Smithsonian Center for Astrophysics: "Astronomers have begun to blast 3 million cubic feet of rock from a mountaintop in the Chilean Andes to make room for what will be the world's largest telescope when completed near the end of the decade. The telescope will be located at the Carnegie Institution's Las Campanas Observatory-one of the world's premier astronomical sites, known for its pristine conditions and clear, dark skies. Over the next few months, more than 70 controlled blasts will break up the rock while leaving a solid bedrock foundation for the telescope and its precision scientific instruments."

It seems whenever I read an article about something new and great discovered by a telescope, it mentions one of the orbiting sattelite type telescopes.

I can't remember when I last heard from a ground based one, except for routine things as continuously sweeping certain areas of the sky for anomalies, like a space surveillance camera.

Now I don't follow astronomy closely, so my viewpoint is based on what of it gets through to general science news sites.But are huge investments in ground based telescopes like this still worth it compared to the alternative?

Most optical observations are done by the ground-based ones. They're more available (there's more of them than there are of the Hubble), flexible, and they're enormous.

What recent stories are you talking about? Among big media ones, I recall some supernova stuff (ground-based telescopes), dark matter stuff in Abell 520 (ground-based Canada France Hawaii telescope and Hubble), and of course planet stuff (ground-based telescopes). Of stuff that doesn't make it to the media, the bulk of optical observing comes from the ground-based ones.

For the same price as a large space-based telescope, you can build a much larger ground-based one. You have to contend with atmosphere (which is why they're always on mountains), but we're getting better at dealing with that. Space-based telescopes make a trade-off: less light gathering as they're smaller, but potentially higher resolution/clarity as you don't have to deal with the atmosphere.

Non-optical telescopes are different though. The atmosphere is quite opaque to much of the non-optical spectrum; far-infrared, X-Ray, and Gamma Ray instruments do best outside the atmosphere. The microwave observatories are out there as well, though I think that's more to avoid noise than atmospheric opacity, as the CMB is very weak.

What you're referring to is interferometry. I came across a news in the last few months that someone somewhere making suggestions along similar lines.
As has already been said, space-based observation is comparatively costlier both to build (and send it up) and difficult to maintain or upgrade compared to ground-based installations.

You're talking about interferometry - you improve your resolution at the expense of field of view, but you don't get any more light gathering power. So it's good if you want to look at very small things, but not so good if you want to look at very dim things, or a lot of the sky at the same time.

Very long baseline interferometry is done with radio telescopes on Earth (and soon in space) that might be on different sides of the planet. You can do this because someone figured out how to do interferometry at

I second this -- IIRC they are doing some enormously clever things with sampling and real-time computation to dynamically eliminate atmospheric distortion, in some cases by using two or more scopes looking at the same thing and in others by basically dynamically deconvolving the distortion to a stationary image. There is a resolution trade-off, not just brightness trade-off, in the size of the primary versus the atmosphere, and where we can (perhaps) compensate for the latter the former is hard physics.

You have to contend with atmosphere (which is why they're always on mountains)

... except for the ones that are not. Which are principally radio telescopes.

To extend your discussion... in addition to ground based telescopes having the potential to be much bigger than space telescopes (the colloquial description is a "light bucket"), they also have the advantages such as being able to receive consumables (e.g liquid helium for cooling IR telescopes, despite being at the bottom of a IR-dirty atmosphere) ; ne

It seems whenever I read an article about something new and great discovered by a telescope, it mentions one of the orbiting sattelite type telescopes.

Putting a telescope in space usually requires some governmental involvement (resources, rockets and/or funding). If there are any issues with a space deployed telescope then you're going to need to perform maintenance via spacewalk (e.g., Hubble).

The results of a space telescope are usually better, but they are smaller, much more expensive and need to get past a lot of obstacles.

plenty of planets have been found by ground based scopes, the closest planet to earth in a star's habitable zone wasn't found by Kepler. And ground based scopes confirm Kepler finds and sometimes are able to perform atmospheric analysis.

With the advances in both active (compensation for deformation of the mirror due to gravity and it's position) and adaptive (compensation of the mirror to negate the effects of atmospheric distortion) optics ground based telescopes can come close to if not equal what can be done in space. When you couple the fact that you can build much larger apertures on the ground for significant less money than what is launched into space I wonder why they are still fooling around with space based telescopes.

Hubble has a 2.4M mirror and cost about 1.5B at launch and over it's lifetime a total of about 6B when you figure in all of the shuttle trips for maintenance and the ground support costs. The 10M Keck telescopes cost 94M each when they were built. The James Webb telescope has become a CF of huge proportions with an estimated cost of close to 8.8B through 2018.

Don't get me totally wrong here. Some magnificent discoveries were made with Hubble along with Swift and Chandra. We do need some space based telescopes but the cost of space base instruments is enormous compared to ground based and there are significant advancements that are being made with sensors and other ground base instruments that are pushing the need for space base instruments further out of the picture.

Active and adaptive optics help compensate for the factors you mention. But they can't do a thing for wavelengths that don't make it through the atmosphere (UV, deep IR, and so on), and those wavelengths are critical for many modern paths of inquiry.

You're absolutely right about the huge expense of space-based platforms, though, to my ongoing dismay. Growing up in the late 60's/early 70's, I expected so very much more of our near future in space.

This is largely an illusion. Most space-based telescopes are run either by NASA or ESA, and both of those organizations have very large public relations offices. These offices issue a lot of press releases and put a lot of effort into getting results from their satellites into the media. The Space Telescope Science Institute was one of the pioneers of this approach to popularizing astronomy, and they were very successful at it. Ground-based observatories tend not to have big public outreach budgets, and usually do not have large numbers of people dedicated to getting their results into the media, so we do not see their results on the front pages of the New York Times or the Economist as often.

Space- and ground-based observatories generally do very different things and complement each other instead of compete with each other. For example, I have used ground-based observatories to take spectra of very faint sources and combined them with X-ray, ultraviolet, and optical observations from Swift and Hubble. The science that comes out of these observations would be impossible without observatories both on the ground and in orbit.

And even if it is completed before TMT and E-ELT, as soon as either of them is completed, it'll lose the title.

Did I mention both TMT and E-ELT are also targeting completion by the end of the decade? Yup.

So, good luck, GMT!

(And it goes without saying that non-optical radio telescopes, which use dishes instead of mirrors, have long been much larger. And that even submillimeter telescopes, which also use dishes, are working on staying larger, with the 25-meter CCAT [ccatobservatory.org] planned for Chile later this decade.)

Yep, of course. In fact, the telescope I run at my job had "the largest monolithic mirror ever made" from 1999 until 2004 (when Roger Angel started cranking out 8.4-meter ones). Wasn't considered the largest telescope, of course, because segmented 10-meter mirrors of the Keck twins (1992 and 1996) next door to it were larger overall, just segmented.

That said, using the phrase "world's largest" in the headline before the first concrete pour invites comments like mine.;)

And these telescopes will be dwarved by future ones. One hundred years from now, we'll probably having giant space telescopes working as a single interferometer with extremely long baselines orbiting the sun beyond jupiter orbit capable of imaging extraterrestial planets.

The media coverage of the dedication ceremony for the scope I run quoted the Minister of Something-or-other as saying something along the lines of "if we asked for this much money now, we'd never get it." The national economy had just peaked when they started building it, and by the time it was finished almost a decade later, the economy sucked.

(And at $400 million, it's still the most expensive scope on the planet, twelve years into its lifespan.)

Maybe they are taking after the USB committee. What are you supposed to do after you give the first revision a name that can't be topped, like Full Speed. My first USB hub was slower than I intended because I thought Full Speed was faster than Hi-Speed.

This is hardly the largest telescope being built. At seven 8.5m mirrors, it is equivalent of a 22m telescope. Just last week I was using the Parkes 64m telescope which was build 50 years ago (and it is hardly the biggest telescope in the world).

Obviously you have never done basic mathematics.... 8.5 x sqrt(7) is 22 (close enough). Why sqrt? because a telescope is (essentially) 2 dimensional - we care about the collecting area not the diameter (directly). A 20m telescope has 4 times the collecting area of a 10m telescope. This is really, really simple stuff.

Physical optical resolution depends on the diameter (directly). Larger area equals more brightness. Larger diameter means smaller angular resolution in the limit, IF one can compensate for other resolution limiting factors, e.g. gravitational and thermal deformation of the mirror(s) and atmospheric effects. Nowadays I believe they largely can, so bigger (larger diameter) is really better, as well as brighter.

Are you suggesting the 7 mirrors will be all next to one another in a long line with 58 meters of length?
If so, you might want to look here [gmto.org] to get a better idea of how it will look like.
And you may want to go here [gmto.org] to see it will actually have a 24.5 meter diameter, which is a lot closer to 22 than to 58.

But hey, don't let facts or the possibility of learning something get in your way... every man deserves the chance to call someone else an idiot.

The "holes" gain you resolution at the expense of longer exposure time requirements. That's the whole meaning behind long baseline interferometry. True a single solid mirror is the best for resolution and exposure, but gaining resolution by using interferometry is an acceptable compromise in astronomy.

Will no one think of the mountains? These ancient citizens of the planet, taking millions of years to form, just minding their own business, and we arrogant humans go and blow the top off of them. Honestly, it's just rude.

I have a question: how come no one has considered using a spot on Australia's Nullarbor Plain to build a giant telescope? Like Chile's Atacama Desert, the Nullarbor Plain has just about no rain and has effective freedom from light pollution, so it would be perfect for a large optical telescope installation.

Wasn't a better approach to building telescopes to have multiple smaller ones working in conjunction, spread out across acres of land (or more) ?

There are two factors that help a large telescope for astronomical observations, resolution and light gathering. Combining smaller scopes (through some process like interferometry) gets you better resolution. There's a limit to what you can do effectively, though and a lot of gear in between has to stay aligned properly to work it. My gut feel for the engineering of it says that the probability of something failing would go up with the square of the number of scopes.
For the other factor, the light gat

Extended and ongoing environmental impact studies are part and parcel of the final process of choosing a telescope site these days. We do try to be very careful to be considerate when building these facilities & work with teams of local environmental biologists. Also, in most cases nowadays, one of the preconditions for site use is that the site is returned to its original pristine state once the telescope in finally removed again.

There is, however, sometimes still local opposition. E.g. in Hawaii, this is usually on native cultural grounds, & nowadays, an effort is made to involve local native cultural leaders (in the early days, some culturally insensitive decisions were made, and both sides of the debate are aware of this).

In the case of Chile, such facilities are often welcomed, as they're much less damaging than the extensive mining operations already in existence there, but still provide good engineering and other technical jobs for Chileans. Plus, many Chileans are proud that their country can boast some of the finest such research facilities in the world.

(Full disclosure: I am both a professional observatory staff astronomer and a longtime member of Greenpeace).